Starch-based adhesives have been used extensively in the production of corrugated paperboard and corrugated containers. Conventional starch-based corrugating adhesives include cooked or gelatinized starch and raw, i.e., unmodified starch particles. The cooked starch functions as a carrier and serves to suspend the unmodified starch granules. The pH of such adhesives is 10.0 of greater, preferably 12.0, in order to reduce the gelation temperature of the raw starch. Typically, borax is included of speed gelation. Gelation, along with the interaction of borax, caustic, and carrier, causes the compositions to function as adhesive upon heating during the corrugation process. The high pH and subsequent solubilization of the unmodified starch after heating result in the poor water resistance of conventional starch adhesives.
U.S. Pat. No. 4,339,364 to Krankkala discloses an aqueous starch-based adhesive comprising starch, a carboxylated styrene-butadiene latex, thickener and a cross-linking agent. The resulting adhesive has excellent water-resistance and has been proposed for use in the same applications and storage systems with conventional starch adhesives. However, as the given pH range at which this adhesive is to be used is 4.5-7.0, there results a high gelation temperature and also it is necessary that the starch mixing equipment be flushed free of any conventional caustic adhesive before this adhesive can be prepared.
Further, prior starch-based corrugating adhesives, including those described in the Krankkala patent cannot be used to bond liner or medium substrates when such substrates have normally adherent contact surfaces at the points where the liner and medium are to be adhered. Thus, if the medium or liner surface is impregnated with a material, such as a waxy or a resinous material, or if the surface otherwise has a low porosity possibly due to the presence of a coating or impregnating material rendering it substantially adherent to conventional or high amylose starch carrier corrugating adhesives at the points of contact, conventional or high amylose starch carrier corrugating adhesives are unsuitable and this is particularly true in the production of the single face bond.
An adhesive composition in accordance with the invention was compared in the laboratory with a conventional corrugating adhesive and also with an adhesive as disclosed in the Krankkala patent with regard to its ability to bond to liner which had been coated with a resin which caused the liner to be highly adherent to conventional or high amylose starch carrier corrugating adhesives and which furthermore had a very low porosity. In these tests, the adhesives were applied to the flute tips of 4".times.6" pieces of single faced board, then brought into contact with the adherent liner and then heated on a 350.degree. F. hot plate in the conventional manner for simulation of the double face bonding process. A 7 pound weight was applied for 6 seconds. The bonds were then pulled apart at different predetermined times and the time required for fiber tear to develop noted. Fiber tear required only 4-6 seconds with the adhesive of this disclosure. Fifteen to twenty seconds were required for the adhesive of the Krankkala patent. Fiber tear never did develop with the conventional or high amylose starch carrier corrugating adhesives.
It was also found that a highly adherent liner which could only be run at 80-120 fpm on a corrugator single facer using high amylose starch carrier adhesives and only 200-300 fpm using the Krankkala adhesive and other specialty adhesives could be run at speeds of 600 fpm when using an adhesive of the present invention. This is thought to derive from the unique properties imparted to the adhesive of the present invention by the interaction of the casein with the carboxylated styrenebutadiene latex.